Request Compression PR 3/3 - Codegen (#3638)

PR 3 of 3. Most of the compression tests happen here since we don't have
a request-compression-supporting model in our set of "smoke test"
service models.

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

---------

Co-authored-by: ysaito1001 <awsaito@amazon.com>
Co-authored-by: Aaron Todd <aajtodd@users.noreply.github.com>